The present invention relates to scanning video imaging systems and, more particularly, to electronically stabilizing a video image having vibration-induced jitter.
Image stabilization is essential to all scanning video image systems. In one particular example, a system is operated under vigorous vibrational conditions, such as occur in a helicopter environment. The system includes a scanner in which the scanning speed may vary and thus cause the video image to jitter. Furthermore, the scanner is located inside a turret, which has a servo loop that is unable to hold the turret at its selected position. This results in the actual turret line of sight jittering about the desired turret line of sight. Because of scanner scan position jitter and turret line of sight position jitter, a different outside scene, rather than the nominal scene, images on the detector.
Since system active sampling periods are generated from internal timing references independent of the jitter, the system active sampling window may start when the scanner is not at its expected starting position, and when the turret is not at the desired turret line of sight. This causes an image shift. Moreover, the incoming detector scene may also be distorted due to instantaneous scanner scan position jitter. The combination of these distortions produces a distorted frame, and the integral of these distorted frames produces a distorted picture. Furthermore, the jitter is two-dimensional. Picture distortion occurs in both vertical and horizontal directions.
Heretofore, several attempts have been made to solve the scanner jitter problem. One approach relied on the generally constant frequency of the master clock to govern the sampling process. The method thus sought to alter the frequency of the system master clock according to scanner speed. This was done by locking the system master clock to the scanner speed by means of a phase locked loop. By doing so, periods of active sampling could vary according to scanning speed variation, and hence stabilize the video picture. This approach required scanners with tight tolerance structures which increase system initial cost and life cycle cost. Moreover, the method could only handle jitter frequencies much below the bandwidth of the phase locked loop.
Another approach relied on the existence of active sample pulses controlling image sampling on a frame to frame basis. This approach altered current frame active sample pulses based on scanner speed measurements collected from previous frames. This method was based on the assumptions that the scanner scanning speed was relatively stable from frame to frame, and that the scanning speed would follow the same accelerating and decelerating paths in each frame. Since these schemes did not compensate for scanner jitter on a real time basis, they did not work well. Under vigorous vibration neither of the two approaches performed as desired.
In regard to turret line of sight jitter, the servo errors are very difficult to compensate. They are usually regarded as being uncompensatable in the scanning system. They are treated as mechanical jitter which should be compensated for by the servo engineers. Turret jitter has large jitter amplitude and high jitter frequencies. Turret line of sight jitter is usually compensated for by electromechanical devices.
One prior attempt to compensate for turret jitter employed a bi-axial high speed, high resolution electro-optical compensation mirror. This biaxial mirror was compact in size and provided compensation in both the azimuth and the vertical direction. However, it was extremely expensive. A bi-axial electro-optical compensation mirror costs ten times as much as a single axis mirror. Furthermore, due to the nature of the bi-axial compensation mirror, the video image or picture produced on the monitor rotates about its own center when the bi-axial compensation mirror is used. This phenomenon becomes more and more obvious as vibration gets greater.
To avoid these disadvantages, a dual single axis mirror was employed for jitter compensation. This mirror arrangement had separate mirrors for azimuth and vertical compensation. Jitters in the azimuth and vertical direction were compensated independently. This solved the image rotation problem and it was also less expensive than the bi-axial mirror. However, the dual single axis mirror requires more electronics and requires a larger turret size. Furthermore, the dual single axis mirror is less reliable.
Accordingly, it is an objective of the present invention to provide all electronic jitter compensation for a video imaging system that will compensate both for servo position residual error caused by turret motion jitter and for scanner motor jitter.